Process for producing expanded polyethylene-based resin beads and process for producing polyethylene-based molded resin object by in-mold foaming

Abstract

A method for producing polyethylene-based resin foamed particles includes a first-step foaming process. The first-step foaming process includes: producing an aqueous dispersion by dispersing polyethylene-based resin particles in an aqueous dispersing medium in a sealed vessel, adding a carbon dioxide-containing foaming agent to the aqueous dispersion in the sealed vessel, heating and pressurizing the aqueous dispersion in the sealed vessel, and releasing the aqueous dispersion in the sealed vessel to a pressure region where a pressure is lower than an internal pressure of the sealed vessel. The foaming ratio in the first-step foaming process is 10 to 18 times. The polyethylene-based resin particles have a polyethylene-based base resin and a melting point of 105 to 125° C., a tan δ of 0.3 to 0.7, and a complex viscosity of 5000 to 20000 Pa·s.

What is claimed is: 
     
         1 . A method for producing polyethylene-based resin foamed particles comprising a first-step foaming process that comprises:
 producing an aqueous dispersion by dispersing polyethylene-based resin particles in an aqueous dispersing medium in a sealed vessel;   adding a foaming agent containing carbon dioxide to the aqueous dispersion in the sealed vessel;   heating and pressurizing the aqueous dispersion in the sealed vessel; and   releasing the aqueous dispersion in the sealed vessel to a pressure region where a pressure is lower than an internal pressure of the sealed vessel,   wherein a foaming ratio in the first-step foaming process is 10 to 18 times,   wherein the polyethylene-based resin particles comprise a base resin that is a polyethylene-based resin,   wherein the polyethylene-based resin particles have a melting point of 105 to 125° C., a tan δ of 0.3 to 0.7, and a complex viscosity of 5000 to 20000 Pa·s, and   wherein the tan δ and the complex viscosity are determined by a viscoelasticity measurement at a temperature of 130° C. and a frequency of 1.67 Hz.   
     
     
         2 . The method according to  claim 1 , wherein the tan δ is 0.4 to 0.6 and the complex viscosity is 6500 to 12000 Pa·s. 
     
     
         3 . The method according to  claim 1 , further comprising a cross-linking process that cross-links the polyethylene-based resin particles. 
     
     
         4 . The method according to  claim 3 , wherein the cross-linking process is performed using a cross-linking agent to cross-link the polyethylene-based resin particles in the aqueous dispersing medium. 
     
     
         5 . The method according to  claim 3 , wherein the cross-linking process is performed before the first-step foaming process. 
     
     
         6 . The method according to  claim 3 , wherein a difference in melting point between the base resin and the cross-linked polyethylene-based resin particles has an absolute value of 2° C. or less. 
     
     
         7 . The method according to  claim 6 , wherein the difference in melting point between the base resin and the cross-linked polyethylene-based resin particles has an absolute value of 1° C. or less. 
     
     
         8 . The method according to  claim 1 , wherein the base resin has a melt index of 0.2 to 2.0 g/10 min. 
     
     
         9 . The method according to  claim 1 , wherein the base resin has a density of 0.920 to 0.932 g/cm 3 . 
     
     
         10 . The method according to  claim 1 , wherein the polyethylene-based resin foamed particles have a melting point of 113 to 117° C. 
     
     
         11 . The method according to  claim 1 , further comprising a second-step foaming process after the first-step foaming process, the second-step foaming process comprising:
 placing the polyethylene-based resin foamed particles obtained by the first-step foaming process in a pressure vessel;   impregnating the polyethylene-based resin foamed particles with an inorganic gas to apply an internal pressure; and   heating and further foaming the polyethylene-based resin foamed particles,   wherein the inorganic gas comprises one or more selected from a group consisting of air, nitrogen, and carbon dioxide.   
     
     
         12 . A method for producing a polyethylene-based resin in-mold foam molded product, the method comprising:
 filling a mold with the polyethylene-based resin foamed particles obtained by the method according to  claim 1 ; and   molding the polyethylene-based resin foamed particles by in-mold foam molding.   
     
     
         13 . The method according to  claim 12 , further comprising, before filling the mold:
 placing the polyethylene-based resin foamed particles in a pressure vessel; and   impregnating the polyethylene-based resin foamed particles with an inorganic gas to apply an internal pressure,   wherein the inorganic gas comprises one or more selected from a group consisting of air, nitrogen, and carbon dioxide.   
     
     
         14 . The method according to  claim 12 , wherein the polyethylene-based resin in-mold foam molded product has a density of 20 to 35 g/L and an amount of water absorption of 0.15 g/100 cm 3  or less. 
     
     
         15 . The method according to  claim 12 , wherein the polyethylene-based resin in-mold foam molded product is a returnable box.